A Mesozoic carbonate hot-spring deposit in the Hartford Basin of Connecticut

Limestone at Coe's Quarry in North Branford, Connecticut, is characterized by a variety of cross-cutting, discontinuous textural facies. The environmental significance of most textural features is ambiguous, but cellular tufa and abundant spherulite strongly suggest a boiling hot spring setting. The cellular tufa consists of a thin, polygonal framework of micritic calcite which surrounds and interconnects irregular elongate voids. The void space is filled with spherulites and locally derived carbonate detritus cemented by sparry calcite and dolomite. Spherulites nucleated and grew in suspension to diameters of 0.1 mm or more before settling in the cellular tufa and other nearby depositional settings. Spherulites may form distinct geopetal fabrics in the pores of the cellular tufa, demonstrating that some indistinct layering in tufa had high primary dips. Other important depositional facies are algal/bacterial tufa, micritic and banded travertine, and siliciclastic, limy sandstone. All units are abruptly discontinuous laterally and are also found as intraclasts mixed in with other facies. The spherulites indicate rapid precipitation of carbonate from highly supersaturated bicarbonate waters formed by flash-boiling of CO (sub 2) -rich water. The cellular textures are similar to microterrace deposits and encrusted organic grains and mats in modern hot springs. Saddle dolomite, which is an important early cement, suggests temperatures of at least 60 degrees C. We infer that the limestone was deposited from a hot spring which periodically flash-boiled in violent, geyserlike eruptions. The Talcott Basalt, which immediately underlies the limestone, is, in places, totally replaced by ferroan carbonates. We infer that the same circulating groundwater, heated by deeper, intrusive equivalents of the Talcott Basalt, locally altered the basalt and reached the surface at hot springs like the one from which the limestone at Coe's Quarry precipitated.